A positive photoresist composition commonly used is a composition comprising an alkali-soluble resin and a naphthoquinonediazide compound as a photosensitive material. Examples thereof include a "novolak-type phenol resin/naphthoquinonediazide-substituted compound" described in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470 and a "novolak resin comprising cresol-formaldehyde/trihydroxy-benzophenone-1,2-naphthoquinonediazide sulfonic acid ester" as a most typical composition, described in L. F. Thompson, Introduction to Microlithography, No. 2, 19, pp. 112-121, ACS Publishing.
In these positive photoresists fundamentally comprising a novolak resin and a quinonediazide compound, the novolak resin exhibits high resistance against plasma etching and the naphthoquinonediazide compound acts as a dissolution inhibitor. The naphthoquinonediazide generates a carboxylic acid on irradiation of light and loses its dissolution inhibiting ability to thereby elevate the alkali solubility of the novolak resin.
From this viewpoint, a large number of positive photoresists comprising a novolak resin and a naphthoquinonediazide-base photosensitive material have heretofore been developed and used in practice, and satisfactory results can be obtained in the working for a line width of approximately from 0.8 to 2 .mu.m.
However, integrated circuits are being more and more intensified in the integration degree and the production of a semiconductor substrate such as VLSI requires working of an ultrafine pattern comprising lines having a width of a half micron or less.
According to one of known techniques for achieving miniaturization of a pattern, a resist pattern is formed using an exposure light source having a shorter wavelength. This technique can be described using the following Rayleigh's formula showing the resolution R (line width) of an optical system: EQU R=k.cndot..lambda./NA
(wherein .lambda. is a wavelength of the exposure light source, NA is a numerical aperture of the lens and k is a process constant). As is apparent from this formula, a higher resolution, namely, a smaller R value can be obtained by reducing the wavelength .lambda. of the exposure light source.
For example, in the production of a DRAM having an integration degree up to 64 M bits, the i beam (365 nm) of a high-pressure mercury lamp is used at present as the light source. In the mass production of 256-M bit DRAMs, use of a KrF excimer laser (248 nm) in place of the i-line has been studied. Further, for the purpose of producing DRAMs having an integration degree of 1 G bits or more, a light source having a further shorter wavelength has been investigated. To this effect, an ArF excimer laser (193 nm), an F.sub.2 excimer laser (157 nm), an X ray, an electron beam and the like are considered to be effective (see, Takumi Ueno et al., Tanpacho Photoresist Zairyo -ULSI Ni Muketa Bisai Kako- (Short Wavelength Photoresist Material -Fine Working Toward ULSI-), Bunshin Shuppan (1988).
When a conventional resist comprising a novolak resin and a naphthoquinonediazide compound is used for the pattern formation by photolithography with a far ultraviolet ray or excimer laser beam, the novolak resin and naphthoquinonediazide compound exhibit strong absorption in the far ultraviolet region and the light scarcely reaches the bottom of resist, as a result, the resist has low sensitivity and only a tapered pattern can be obtained.
One of the techniques for solving this problem is the chemical amplification-type resist composition described in U.S. Pat. No. 4,491,628 and European Patent No. 249,139. The chemical amplification-type positive resist composition is a pattern formation material which generates an acid in the exposed area on irradiation of radiation such as far ultraviolet ray and due to the reaction using the acid as a catalyst, differentiates solubility in a developer between the area irradiated with the active radiation and the nonirradiated area to form a pattern on a substrate.
Examples thereof include combinations of a compound capable of generating an acid by photolysis with an acetal or O,N-acetal compound (see, JP-A-48-89003 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), with an ortho ester or amide acetal compound (JP-A-51-120714), with a polymer having an acetal or ketal group on the main chain (JP-A-53-133429), with an enol ether compound (JP-A-55-12995), with an N-acyliminocarbonic acid compound (JP-A-55-126236), with a polymer having an ortho ester group on the main chain (JP-A-56-17345), with a tertiary alkyl ester compound (JP-A-60-3625), with a silyl ester compound (JP-A-60-10247) or with a silyl ether compound (JP-A-60-37549, JP-A-60-121446). These combinations in principle have a quantum yield exceeding 1 and therefore exhibit high photosensitivity.
A system which decomposes by heating in the presence of an acid and is alkali-solubilized is also used and examples thereof include combinations of a compound capable of generating an acid on exposure with an ester or carbonic acid ester compound having a tertiary or secondary carbon (e.g., tert-butyl, or 2-cyclohexenyl) described, for example, in JP-A-59-45439, JP-A-60-3625, JP-A-62-229242, JP-A-63-27829, JP-A-63-36240, JP-A-63-250642, JP-A-5-181279, Polym. Eng. Sce., Vol. 23, page 1012 (1983), ACS. Sym., Vol. 242, page 11 (1984), Semiconductor World, November, 1987, page 91, Macromolecules, Vol. 21, page 1475 (1988), and SPIE, Vol. 920, page 42 (1988), with an acetal compound described, for example, in JP-A-4-219757, JP-A-5-249682 and JP-A-6-65332, or with a tert-butyl ether compound described, for example, in JP-A-4-211258 and JP-A-6-65333.
Such systems are mainly composed of a resin having a basic skeleton of poly(hydroxystyrene) which is small in the absorption in the region of 248 nm and therefore, when the exposure light source is a KrF excimer laser, they have high sensitivity and high resolution and are capable of forming a good pattern. Thus they can form good systems as compared with conventional naphthoquinonediazide/novolak resin systems.
However, when the light source has a still shorter wavelength, for example, when the exposure light source used is an ArF excimer laser (193 nm), the above-described chemical amplification systems are yet deficient because the compound having an aromatic group substantially has large absorption in the region of 193 nm. As a polymer having small absorption in the 193 nm region, in J. Vac. Sci. Technol., B9, 3357 (1991), the use of poly(meth)-acrylate is described. However, this polymer has a problem in that the resistance against dry etching which is commonly performed in the production process of semiconductors is low as compared with conventional phenol resins having aromatic groups.
In Proc. of SPIE, 1672, 66 (1922), it is reported that polymers having alicyclic groups exhibit the dry etching resistance on the same level as that of the compounds having aromatic groups and at the same time, have small absorption in the 193 nm region. The use of these polymers has been aggressively studied in recent years. Specific examples thereof include the polymers described, for example, in JP-A-4-39665, JP-A-5-80515, JP-A-5-265212, JP-A-5-297591, JP-A-5-346668, JP-A-6-289615, JP-A-6-324494, JP-A-7-49568, JP-A-7-185046, JP-A-7-191463, JP-A-7-199467, JP-A-7-234511, JP-A-7-252324, JP-A-8-259626, JP-A-9-73173 and JP-A-9-90637. However, these polymers do not always have sufficient dry etching resistance and are disadvantageous in that the synthesis thereof necessitates many steps.
Further, the introduction of the alicyclic group in order to improve the dry etching resistance is accompanied with the decrease in adhesion to a substrate and causes a problem in that peeling off of the resist layer occurs in the pattern after development. In order to solve such a problem, a resin containing a unit having a .gamma.-butyrolactone structure is proposed as described in EP-A-856773. Although the adhesion is improved by using the resin, it has only a low acid decomposing property due to the secondary carboxylic acid ester, resulting in decrease in sensitivity and exhibits insufficiently low resolution. Further, since the hydrophobic property of the resist layer increases by the introduction of the alicyclic group, such a resist composition has another problem of the occurrence of development defects.